In recent years, the data recording density in magnetic disk devices has been remarkably increased. The rate of increase in recording density in magnetic disk devices is said to be 100% per year. With the remarkable increase in data recording density, a quantum leap has been achieved in data recording capacity of magnetic disk devices. The increase in data recording capacity has accelerated the reduction in size of magnetic disk devices, and cost-effective magnetic disk devices have also been made practical which may replace semiconductor memories in various electronic devices such as cameras, facsimile machines, portable telephones, modems, pagers, handheld computers, printing machines and copying machines.
An actuator arm of a magnetic disk device has a slider attached to its distal end, and a read/write magnetic head is incorporated in the slider. The magnetic head is ordinarily placed in the vicinity of an air flow outgoing end of an air bearing surface of the slider facing a disk. An air flow generated with the rotation of the disk is drawn into the gap between the air bearing surface and the disk surface to float the slider above the disk. Thus, the slider floats above the rotating disk. The floating altitude is the thickness of the air lubrication film, i.e., the distance between the disk surface and the slider.
Thus, an air bearing surface is formed in the surface of the slider facing the disk, and a self-pressurizing air lubrication film is formed between the slider and the disk storage surface and is maintained. This film inhibits mechanical contact between the slider and the disk during disk rotation to limit friction and wear.
In magnetic disk devices, the amount of floating of the slider from the disk during recording or reproduction tends to decrease with the increase in recording density. This decrease in the amount of floating is achieved by a negative pressure utilization type of slider in which a slider air bearing surface is formed of a plurality of generally flat surfaces differing in height; a positive pressure is generated at the generally flat surface formed higher so that the gap between the slider and the disk is smaller; and a negative pressure is generated at the generally flat surface formed lower so that the gap between the slider and the disk is larger, and which floats by the positive and negative pressures balancing with each other. Such a negative pressure utilization type of slider has already been disclosed (see, for example, JP1505878B, JP2778518B, and JP2803639B).
In magnetic disk devices today, a rotary actuator similar to a tone arm of a record player is provided in order to obtain a high access speed. If such a rotary actuator is used, slide between the slider and the disk and the air flow below the slider are no longer unidirectional but widely varying in angle with respect to the longitudinal axis of the slider. Also, the high-speed search operation of the actuator during access acts as a cause of inclination of the direction of slide between the slider and the disk and the direction of the air flow below the slider from the longitudinal axis. Therefore, it is no longer thought that in rotary actuator magnetic disk devices in recent years the sliding direction corresponds to the direction of the longitudinal axis of the slider from the front to the rear or deviates only slightly from the longitudinal axis direction.
The angle of the disk sliding direction with respect to the longitudinal axis of the slider is called a skew angle. When the actuator arm is positioned so that the sliding direction passes through an outer end of the slider or a point outside the slider, the skew angle is positive. When the actuator arm is positioned so that the sliding direction passes through an inner end of the slider or a point at a hub, the skew angle is negative.
When data access is made, the slider is moved through a range from the disk inner periphery to the disk outer periphery. In this movement, the amount of floating of the slider and the floating attitude of the slider are changed, for the reason described below. In the rotary actuator magnetic disk device, not only the relative speed between the slider and the disk but also the skew angle changes with respect to the disk radial position, so that the distribution of the pressure of air produced on the air bearing surface is changed. Due to the change in the amount of floating of the slider, the electromagnetic conversion efficiency of the magnetic head is reduced. It is therefore required that, in a magnetic disk device of which high recording density is required, the amount of floating at the magnetic head position be uniform from the disk inner periphery to the disk outer periphery. With the reduction in the amount of floating of the slider, the requirement with respect to variation in the amount of floating becomes stricter.
In the slider of which constant floating at the magnetic head position is required, there is a risk of the amount of floating being reduced at the minimum-floating-amount position due to a change in the floating attitude of the slider to cause contact between the slider and the disk, i.e., a so-called head crash. Therefore there is a need to maintain the floating attitude of the slider with stability.
FIG. 20 shows an example of an air bearing surface 30. The air bearing surface 30 is fixed in a lower surface of a slider and facing a disk. The air bearing surface 30 is formed in a certain configuration by molding, etching, laser cutting, ion crushing, general-purpose machining or any of other various methods. The air bearing surface 30 is constituted of three flat surfaces forming three stages substantially parallel to each other: an upper stage surface 31, a middle stage surface 32 and a lower stage surface 33. In FIG. 20, the upper stage surface 31 is indicated by a blank area, the middle stage surface 32 by coarse hatching, and the lower stage surface 33 by dense hatching. A head 99 is incorporated on the upper stage surface 31 in vicinity to the air out going end 42. Thus, the air bearing surface is formed in a complicated geometric configuration to limit variation in the disk radius position (variation in relative speed) and to limit the change in the amount of floating and to maintain the floating attitude at the position of the head 99 with the change in the skew angle.
As a background to the air bearing surface configuration becoming complicated, the fact may be mentioned that, in air bearing surfaces in recent years, as shown in FIG. 20, the middle stage surface 32 formed on the air flow incoming end 41 side of the air bearing surface 30 is extended to the air flow incoming end 41 and, therefore, the effect of tapering disclosed in U.S. patents (see, for example, U.S. Pat. Nos. 4,673,996 B, 5,404,256 B and 5,936,800 B) is reduced and the amount of air inflow to the air bearing surface 30 is limited. The arrangement in which the middle stage surface 32 is extended to the air flow incoming end 41 in spite of the reduction in the effect of tapering is adopted for the purpose of limiting the change in the amount of floating due to variation in thickness of a wafer provided as a substrate for the slider. Variation of +/−20 um exists in the wafer thickness. Therefore, if the lower stage surface 33 is formed at the air flow incoming end 41 to obtain the effect of tapering, the length of the lower stage surface 33 formed at the air flow incoming end 41 varies due to variation in the wafer thickness (the length from the air flow incoming end 41 to an air flow outgoing end 42 when the slider is formed). The amount of air inflow is thereby changed largely, resulting in considerable variation in the amount of floating of the slider (see, for example, JP2001-325707A). FIG. 21 is a citation of a diagram of the relationship between variation in substrate thickness and variation in the amount of floating shown in JP2001-325707A. In FIG. 21, symbol ♦ indicates the amount of floating of a slider having a lower stage surface 33 formed at the air flow incoming end 41, and symbol ▪ U indicates the amount of floating of a slider having no lower stage surface 33 formed at the air flow incoming end 41. It can be understood from FIG. 21 that variation in the amount of floating of the slider having no lower stage surface 33 formed at the air flow incoming end 41 is markedly improved in comparison with that in the case where the lower stage surface 33 is formed at the air flow incoming end 41. The air bearing surface today is formed in a complicated configuration to improve the floating characteristics of a slider, which are reduced by limitation to the effect of tapering. The formation of such a complicated air bearing surface ensures that working unevenness is limited and that a slider having the desired floating characteristics can be provided.
The configuration of the air bearing surface is ordinarily determined by repeating computation using a special-purpose numerical analysis solver capable of analysis of the amount of floating and the floating attitude of the slider. Each time the design of a magnetic disk device is changed, there is a need to change the design of the air bearing surface configuration according to the disk rotation speed, the skew angle and the amount of floating in the magnetic disk device.
Operations for this kind of design change are time-consuming and the designer needs to have knowledge about fluid dynamics and to be skilled in slider design. In recent years, the recording density has been remarkably increased and the model cycle of magnetic disk devices has become shorter. With these changes, the design cycle of the slider air bearing surface has also become shorter. There is a serious problem that the load on designers is increasing.
Magnetic disk devices using small-diameter disks having diameters of, for example, 27 mm and 20 mm are presently under development. The disk diameter of such magnetic disk devices differs largely from that of the conventional magnetic disk device using a disk having a diameter of, for example, 95 mm or 65 mm. Magnetic disk devices using small-diameter disks are incorporated in portable devices in many cases and those designed to reduce the rotational speed of the spindle motor from the viewpoint of power consumption and noise have been put to practical use. The relative speed between the slider and the disk is therefore increased in comparison with that in the conventional design, so that the desired characteristics ensuring stability of the amount of floating of the slider or air film rigidity for example cannot be obtained as long as the slider design techniques accumulated in the conventional design are used. This is also a cause of an increase in the load on designers. In addition, it is greatly important to consider use of portable devices on high ground, and it is required that the change in the amount of floating of the slider be small with respect to a change in atmospheric pressure. It is extremely difficult to ensure the desired atmospheric pressure variation characteristic in the case of using a smaller disk, since the usable amount of air is reduced in comparison with that in the conventional type.
Further, it is also difficult to ensure the desired slider floating characteristics, including an atmospheric pressure variation characteristic, of sliders of a small air bearing surface area such as femto-type sliders, which are presently being introduced, because the amount of air inflow on the air bearing surface is reduced.